Choosing a Battery-Based Inverter

A residential battery-based inverter has a primary task of accepting DC electricity and changing it into standard, household AC electricity. This seems simple enough, but wading through the inverter specifications can be overwhelming if you don’t understand what they mean and how they pertain to your system.

Your system may have other requirements (such as battery charging from an engine generator or the grid), and figuring out which features will be advantageous often depends on the system type and specifics. With no access to utility power, an off-grid system has to supply all of the electricity at all times. A grid-tied battery backup system has access to utility power, can use the inverter to export excess electricity back to the grid, and usually supplies electricity only to specific “critical” loads during a utility outage. These are some of the differences that make certain inverter features desirable in one system type but not necessarily in the other. As you go through the specification descriptions, you will see how these differences influence the inverter selection process.

This guide includes a specifications table for available battery-based, sine-wave inverters that are listed to the Underwriters Laboratories 1741 standard and commonly used in residential applications (2 to 8 kW). The compiled data is from manufacturers and their specifications sheets.

The Specs

Off-Grid, Grid-Tied, or Both tells us what system type(s) this inverter is built for.

Rated Continuous Output Power represents the inverter’s capacity. For example, a 2,000 W inverter is rated to supply 2 kW of AC power continuously. In an off-grid system, this value determines the total wattage limit of AC loads that can be run simultaneously. You must specify an inverter with an output power rating large enough to handle all of your simultaneous AC loads.

Let’s say we want to power the following at the same time:

• a 1,400 W microwave
• six 15 W lights
• a 100 W refrigerator
• a 120 W TV

In this case, an inverter with a continuous output power rating exceeding 1,710 W would suffice (1,400 + 90 + 100 + 120). Surge ratings are discussed separately.

For grid-tied battery-based inverters, the power rating is examined under two scenarios—when the grid is available and when there is an outage. When the grid is up, the inverter’s job is to convert all available DC power from the renewable energy system to AC, which is used in the home. If the array output exceeds household demand, the excess is sent to the utility. The inverter capacity must be large enough to accommodate the RE system size. For instance, an inverter for a 4,000 W PV array will generally be sized at that same power rating. (However, because climate factors such as warm temperatures will limit PV array output, the array-to-inverter ratio may vary.)

When the utility is down, the inverter’s job is to supply power to all the AC loads connected to it. Most of these systems include a “critical load subpanel” so that not all of a home’s loads have to be energized, which keeps battery and system costs down. The inverter capacity must be large enough to meet the total requirement of all connected AC loads that might be run simultaneously, and large enough to handle the RE output. (See “Sizing a Battery-Based Inverter” in the Circuit: Methods in this issue.)